Electromagnetic Waves: JEE Main 2012 Physics question

An electromagnetic wave in vacuum has the electric and magnetic field $\mathop E\limits^ \to$and $\mathop B\limits^ \to$, which are always perpendicular to each other. The direction of polarization is given by $\mathop X\limits^ \to$ and that of wave propagation by $\mathop k\limits^ \to$. Then